1. Field of the Invention
The present invention relates to evaporation donor substrates used for deposition of materials which can be deposited by an evaporation method. In addition, the present invention relates to a method for manufacturing light-emitting devices which employs the evaporation donor substrate.
2. Description of the Related Art
Light-emitting elements which use organic compounds and are characterized by thinness, lightweight, fast response, and direct current low voltage driving, are expected to be applied as a light-emitter to next-generation flat panel displays. In particular, a display device in which light-emitting elements are arranged in matrix has an advantage in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
A light emission mechanism of a light emitting element is described below: when voltage is applied between a pair of electrodes with an EL layer interposed therebetween, electrons injected from a cathode and holes injected from an anode are recombined at an emission center in the EL layer to form molecular excitons, and energy is released when the molecular excitons relax to the ground state. As excited states, a singlet excited state and a triplet excited state are known, and light emission is possible through either of these excited states.
An EL layer included in a light emitting element has at least a light emitting layer. In addition to the light emitting layer, the EL layer can have a stacked-layer structure including a hole-injecting layer, a hole-transporting layer, an electron-transporting layer, an electron-injecting layer, and/or the like.
EL materials for forming an EL layer are broadly classified into a low molecular (a monomer) material and a high molecular (a polymer) material. In general, a film of a low molecular material is often formed by an evaporation method and a film of a high molecular material is often formed by an inkjet method or the like.
An evaporation apparatus which is used in an evaporation method has a substrate holder to which a substrate is mounted; a crucible (or an evaporation boat) containing an EL material, that is, an evaporation material; a heater for heating the EL material in the crucible; and a shutter for preventing the EL material from being scattered during sublimation. The EL material which is heated by the heater is sublimed and deposited onto the substrate.
Note that in order to achieve uniform deposition, actually, a deposition target substrate needs to be rotated and the substrate and the crucible need to be separated from each other by at least a certain distance. In addition, when films of different colors are separately formed using a plurality of EL materials through a mask such as a metal mask, it is necessary that the distance between pixels be designed to be large and that the width of a partition (bank) formed of an insulator between pixels be large. Such needs are major problems in advancing high definition (increasing the number of pixels) and miniaturization of pixel pitches along with downsize of a light-emitting device including a light-emitting element.
Therefore, as for flat panel displays, it has been necessary to solve those problems as well as to achieve high productivity and cost reduction in order to achieve higher definition and higher reliability.
Thus, a method for forming an EL layer of a light emitting element through laser thermal transfer has been proposed (see Reference 1: Japanese Published Patent Application No. 2006-309995). Reference 1 describes a transfer substrate which has a photothermal conversion layer including a low-reflective layer and a high-reflective layer and also a transfer layer over a supporting substrate. Irradiation of such a transfer substrate with laser light allows the transfer layer to be transferred to an element-forming substrate.